This invention relates to an improved energy efficient method of heating and cooling cast metal structures such as ingots and slabs, and to devices suitable for practicing the method.
When an ingot is cast from molten metal, it is first allowed to cool in the mold to a state wherein its outer skin is solid and its interior is very soft or liquid. The mold is then stripped away, and the ingot is placed in a specialized furnace known in the art as a "soaking pit" where it is allowed to cool slowly until it solidifies throughout. Slow cooling is accomplished by subjecting the ingot to the vitiated combustion products of high quality fossil fuels.
Although soaking pits are used to carry out a process of controlled cooling, they are one of the largest consumers of fuel in the steel making industry. Moreover, because of the temperature that must be maintained in the soaking it (on the order of 2200.degree. F. to 2350.degree. F.), the metal being treated is subject to oxidation and chemical attack from stray impurities in the fuel such as sulfur. For this reason, natural gas, low sulfur distillate oil, or some other fuel of equivalent purity must be employed. The fuel consumption of a soaking pit ranges from about 0.5.times.10.sup.6 BTU to about 3.0.times.10.sup.6 BTU per ton of material processed. Given that the steel industry in the U.S.A. produces about 150.times.10.sup.6 tons of steel per year, and that nearly all of this production goes through soaking pits, the annual consumption of fuel for the controlled cooling of steel ingots is equivalent to approximately 363 billion cubic feet of natural gas, or about 2% of the total domestic annual natural gas consumption.
Even though fuel of high chemical purity is used in soaking pits, and despite the control exerted over the combustion process, oxidation of the steel being processed proceeds so rapidly that approximately 4% of its mass is lost as scale during treatment in the soaking pit furnace. Obviously, this alone has serious economic implications for steel making. The composite price of finished steel products is currently about 18 cents per pound or $360.00 per ton. Thus, the loss of material in the soaking pits represents a loss of approximately $5.00 per ton of product. The cost of fuel used to maintain the temperature of the soaking pits also represents about $5.00 per ton. To put these expenses in perspective, it should be noted that the value of the fuel used and of the scale loss is comparable with the profit per ton of finished product reported by several domestic steel companies.
In the subsequent processing of steels and other metals, it is often necessary to heat the metal prior to subjecting it to rolling, forging, annealing, or other hot forming process. For example, in many steel mills, steel slabs weighing on the order of 100 tons each are heated from about 70.degree. F. to 2350.degree. F. prior to undergoing a rolling process. This step is known in the metal processing art as "reheating".
The most widely used reheating apparatus is the combustion furnace wherein the material treated is exposed directly to the flame of the fuel employed. Heat is transferred to the material by direct convection from the combustion products, by radiation from the flames, and by radiation from the ceramic material which lines the furnace. In order to obtain a reasonable level of efficiency, all such furnaces must be equipped with heat recuperation equipment which typically represents as much as two-thirds of their total capital cost. Typically, the recuperator system comprises a gas-to-gas heat exchanger which is notoriously subject of fouling.
These combustion reheating furnaces suffer from a number of disadvantages, perhaps the most serious of which is their low energy efficiency. In this regard, one of the most advanced designs presently available is said to be capable of reheating 300 tons of steel per hour from 70.degree. to 2350.degree. F., at a fuel cost of about 10.sup.9 BTU. Given that the specific heat of steel is about 0.11 BTU/lb/.degree.F., the heat stored in the steel is approximately 1.505.times.10.sup.8 BTU. Thus, the heat transfer efficiency of this rather advanced version of a combustion reheating furnace is only on the order of 25%.
Another disadvantage of combustion reheating furnaces is their inability to be quickly turned on and off, sometimes referred to as their high "thermal inertia". This property results from the inability of the refractory ceramic liners to be heated and cooled rapidly without undergoing serious damage. Thus, in many operations (e.g., stainless steel production) a combustion reheating furnace is in practice energized 24 hours a day, even though it may be used only 40 hours per week. This of course represents a very large burden on operating costs and gives rise to severe maintenance problems.
As in the soaking pit furnaces, another drawback of combustion reheating furnaces results from the need to control the atmosphere within the furnace itself to minimize chemical damage to the material being processed. This necessitates the use of fuels of very high chemical purity such as natural gas, propane, and low sulfur distillate oils. If high sulfur bearing fuels are used, a sulfide scale forms on the stock. Use of a fuel high in alkali ash leads to chemical damage to the stock. Even when using the relatively pure fuels mentioned above, the formation of oxide scale amounts to a considerable material loss and creates an environmental pollutant which must be dealt with. Thus, about 4% of all material passing through a stainless steel production mill is lost as oxide scale in the reheating furnace. Furthermore, scale often causes accelerated wear in forming equipment such as forging dies.
It should be appreciated that there is no significant opportunity to employ the direct combustion of coal in either reheating furnaces or soaking pits since at the combustion temperatures required, the ash of most domestic coals softens and clings to heat transfer surfaces like a paste, thereby rapidly fouling heat exchange surfaces. Also, the alkali ash in the coal can induce adverse chemical effects on the workstock.
Because of the foregoing disadvantages, and with the increasing cost of chemically pure fuels, it has now become attractive to employ electric furnaces for reheating purposes, and in fact, some recently constructed steel plants already employ electric induction reheating. These furnaces offer several advantages, such as enabling the use of coal as a primary fuel (to generate electricity) and providing flexibility in the control of the atmosphere surrounding the workpiece.
In the furnace of these plants, the stock is surrounded by a coil of copper bars which, when energized with AC power, induces eddy currents in the stock itself, resulting in heating by eddy current dissipation. As stock temperature rises, the stock radiates heat to the walls of the furnace which typically consist of the induction coil itself shielded by a thin refractory ceramic plate on the order of one inch thick. As the coil is heated by radiation from the stock and ceramic plate, its resistivity increases and the eddy current losses in the coil become significant. Water is circulated through the coils to prevent them from overheating. Although only somewhat less than half of the electrical energy supplied to the coil is actually transferred to the workstock as heat, the net fuel use efficiency in such electro-induction reheating furnaces is comparable to the most efficient combustion furnaces. Also, the electro-induction furnace can be turned on or off quite rapidly, thereby offering important advantages in flexibility of operation and ease of maintenance. However, currently employed electro-induction furnaces heat the workpiece in an environment of air rather than nearly vitiated combustion products, and scale production takes place rather rapidly. In fact, the production of oxide scale in such furnaces is so rapid that the workstock often undergoes "spalling", a phenomenon wherein flakes of scale are expelled from the surface of the workpiece with such force that they become embedded in the refractory sheath that protects the induction coils.
A typical electro-induction reheating furnace for processing 100 tons of steel per hour has an average power demand on the order of 30 to 31 megawatts. Energy flux into the unit is therefor about 1.05.times.10.sup.8 BTU per hour. To heat 100 tons of steel from 70.degree. F. to 2350.degree. F. required 5.016.times.10.sup.7 BTU. Accordingly, the efficiency of energy transfer to the workpiece in such a furnace is approximately 48%. To put the economics in perspective, it should be noted that the capital costs of a furnace of this type are about 1.5 million dollars. To build the electric generating capacity required to energize the furnace, at its peak load of 35 megawatts, costs approximately 36.4 million dollars. Accordingly, improvements in efficiency in the conversion of electrical energy to heat in the workstock would be economically attractive.